The present invention relates to a method of marking a surface of a diamond or gemstone. The mark may be any mark, but the invention is particularly though not exclusively directed to applying an information mark to the diamond or gemstone. The diamond may be for instance an industrial diamond such as a wire-drawing die-or diamond optical component, though the invention is of particular interest in marking gemstone diamonds, for instance for applying a mark which is invisible to the naked eye or invisible to the eye using a xc3x9710 loupe, when the mark can be applied to a polished facet of the gemstone without detracting from its clarity or color grade. When a loupe is used, the visibility is assessed under the internationally accepted conditions for clarity grading, i.e. using a 10xc3x97 magnifying achromatic, aplanatic loupe under normal light, this being a white diffuse light, not a spot light. The marks can be used to uniquely identify the gemstone by a serial number or as a brand or quality mark. In general, the mark should be capable of being viewed under suitable magnification and viewing conditions, and, if applied to a gemstone, should not detract from the value or appearance of the stone and should preferably not exhibit blackening.
There is a detailed description of the nature of the marks that can be applied in WO 97/03846, which is incorporated herein by reference and in which the marks are applied by irradiating a diamond gemstone with ultraviolet laser radiation using a projection mask. U.S. Pat. No. 4,425,769 describes providing an identifying mark on a diamond or other gemstone by applying a photoresist to the surface, forming a contact mask by a photographic method, and etching the gemstone through the mark by cathode bombardment with an ionised gas to provide sputter etching. Sputter etching gives poor control of the depth of the mark and low resolution.
According to a first aspect of the present invention, the surface of a diamond or gemstone is marked with a focused ion beam, the mark being invisible to the naked eye. The invention extends to a diamond or gemstone which has been marked by the method of the invention, and to apparatus for carrying out the method.
The marking can be carried out by direct writing on the diamond or gemstone surface with a focused ion beam. Typically Gallium ions are used, but a beam of other suitable ions may alternatively be used. By limiting the dose, sputtering of carbon atoms can be substantially avoided, sputtering causing direct material removal; this enables a mark to be applied with a controlled depth and good resolution. By limiting the dose, and providing there is sufficient dose, the incident ions cause disordering of the crystal lattice. In the case of diamond, this converts the diamond to a graphite-like or other non-diamond structure that can then be cleaned, e.g. using an acid or potassium nitrate dissolved in acid, to leave a shallow mark say not less than 10 nm deep and/or not more than 70 nm deep, more preferably say not less than 20 nm deep and/or not more than about 50 nm deep, typically about 30 nm deep, with no evidence of blackening. Plasma etching may be used as an alternative to acid cleaning.
However, in a preferred embodiment, the disordered layer produced on the diamond or gemstone by the ion beam is removed by means of a powerful oxidizing agent, such as molten potassium nitrate. This method allows a mark to be produced at a lower dose and therefore in less time at a given beam current. Alternatively, a lower beam current, giving a smaller spot size may be used to produce marks with higher resolution features, such as diffraction gratings.
The depth of the lattice disordering is determined by the range of the ions. For 50 keV Gallium, this range is about 30 nm. The minimum dose may be as low as 1013/cm2, but is preferably about 1014/cm2 to 1015/cm2. However, good marks can be applied with a fairly modest dose, the preferred maximum dose being about 1016/cm2 or even up to about 1017/cm2. However, the dose depends upon the ions being used and their energy (as measured in keV). The ion beam dose is a total number of incident ions per unit area at the sample surface, during the marking. The beam current may be about 1 nA, and the beam energy not less than about 10 keV or about 30 keV and/or not greater than about 100 keV or about 50 keV.
It has been found that if depth of mark is plotted against ion beam dose for a series of different beam energies, there is an increase of depth of mark with increasing beam energy. Characteristics of the mark may be optimised by selecting from the dose/energy combinations which will result in the desired depth of mark.
The region to be marked and/or the surrounding area may be coated with an electrically-conducting layer, for instance gold, prior to forming the mark, so that an electrical connection can be provided before marking with the ion beam, to prevent charging. The thickness of the gold, or other, coating alters the variation of depth of mark with beam energy and dose, and may thus be chosen to optimise the mark produced.
Other suitable methods to reduce charging may be used. One method is to irradiate the region to be marked with a low energy ion beam, e.g. about 3 to about 10 keV, prior to forming the mark, to modify the diamond surface to cause it to become electrically conductive, the electrical connection being made to that region. In a preferred embodiment, the ion beam used for marking may be used in conjunction with a charge neutralising device, such as an electron flood gun, such as that described in U.S. patent specification number U.S. Pat. No. 4,639,301, to prevent charging of the diamond surface.
In accordance with a second aspect of the present invention, there is provided a method of marking the surface of a diamond or gemstone, comprising the steps of irradiating at least a portion of said diamond or gemstone to form a damaged or crystal lattice disordered layer thereon, and removing said disordered layer using an oxidizing agent.
A further advantage of the second aspect of the present invention over acid-cleaning is that no acid fumes are produced and also that spent acid does not have to be disposed of, thereby improving the safety of the process as well as offering environmental and economic benefits.
The oxidizing agent is preferably molten potassium nitrate. The diamond or gemstone is preferably covered with potassium nitrate and heated to a temperature of around 380-550 Centigrade for a period of between a few minutes and several hours, preferably approximately one hour.
However, other suitable powerful oxidizing agents include molten compounds such as alkali metal salts. Suitable compounds may be in the form Xn Ym where the group X may be Li+, Na+, K+, Rb+, Cs+, or other cation, and the group Y may be OHxe2x88x92, NO3xe2x88x92, O22xe2x88x92, O2xe2x88x92, CO32xe2x88x92, or other anion; the integers n and m being used to maintain charge balance. Mixtures of compounds may be used. Air or other oxygen-containing compounds may also be present.
The use of such oxidizing agents to remove a disordered layer allows a mark of a desired depth to be produced using a relatively low dose of ions.
In a preferred embodiment, the diamond or gemstone is irradiated with an ion beam as in the first aspect of the present invention, and most preferably a Gallium ion beam. The preferred embodiment of the method of the second aspect resulting in a remarkably efficient process, with each incident Gallium ion ultimately resulting in the removal of approximately 2,700 carbon atoms. In most materials other than diamond, this figure would be around 1-10.
It is this property of diamond that allows the relatively large structures such as alphanumeric characters covering an area of 0.43 mm by 0.16 mm to be machined in a reasonably economic time of about 10 seconds.
The method of the present invention may also be used to mark the surface of a synthetic gemstone, such as the silicon carbide gemstones described in WO 97/09470.